Acid-activated clays of high activity are used to adsorb colored pigments (carotenoids, chlorophyll) and colorless pigments (phospholipids) from edible and inedible oils. This process is called "bleaching" and serves both cosmetic and chemical purposes. Thus, bleaching reduces color, whereby very clear, almost water white oils are produced that meet with consumer expectations. Bleaching also stabilizes the oil by removing colored and colorless pigments which tend to "destabilize" the oil, resulting in oils that rancidify more easily if they are not removed. The current, and expected long term trend, favors the use of highest possible bleaching efficiency clays with this process because: (i) the smaller amounts of the high activity clays needed to produce desired refined oil properties mean that lower inventories can be maintained by the oil refiner; (ii) refined oil losses depend somewhat on the amount of clay used because less of high activity clay needs to be used and therefore oil losses are lower; and (iii) less spent clay is produced when using high activity clay, and hence land-fill disposal costs are lower.
The conventional process for producing acid-activated bleaching clays utilizes calcium bentonite clays and requires relatively high acid dosages to achieve maximum bleaching efficiencies. The calcium bentonites used in the process are hydrated sodium calcium aluminosilicates which typically are mildly basic. The manufacture of highest quality commercial bleaching earths typically require 70-90 grams of 96% H.sub.2 SO.sub.4 /100 grams of dry clay or 67.2-87.4% acid dosage where: ##EQU1## Extensive leaching of the clay structure in the form of solubilized salts takes place and these are removed in the process. Because of these high acid dosages, and the extensive leaching that takes place during the leaching process, the yield of bleaching clay is low (typically in the range of 75-85 wt %). The acidic salts formed during activation and residual acid must be washed off and separated by filtration from the product clay. If high levels of unused acid and acidic salts (iron and aluminum sulfates) are left in the clay, the quality of the bleached oil is impaired. High residual acid levels generate undesirable free fatty acids from the fatty acid triglycerides in the oil. Finally, the leachate (acidic salts and residual acid) is a waste stream that contains materials harmful to aquatic life and therefore must be neutralized or otherwise disposed of in an environmentally acceptable manner. This constitutes an additional expense of producing bleaching clays from pure calcium bentonites.
Clay sources used in the past to provide acid-activated bleaching clay of high activities have been primarily restricted to calcium bentonites, i.e., clays in which the principal exchangeable cation is a calcium ion, and these are sometimes also referred to as sub-bentonites.
Another type of naturally-occurring clay simply requires heat to impart bleaching activity. These are the clays rich in the minerals attapulgite or sepiolite, now frequently classified as palygorskite clays. Mineralogically, the palygorskite clays are readily distinguishable from the bentonites (smectites or montmorillonites) and rarely are palygorskites and bentonites used interchangeably.
It has been the general belief that palygorskite clays do not respond to the conventional acid-activation treatment. The same is true of certain bentonites, namely sodium (swelling) bentonites, such as Wyoming bentonites.
The following publications pertain to the art of preparing bleaching earths from naturally-occurring clays.
A. D. Rich, "Bleaching Clay", Industrial Rocks & Minerals, 3rd. Ed., AIME, N.Y. pp 92-101 (1960). PA1 R. Fahn, "Bleaching Earths-Preparation, Properties, Practical Applications", Chapter 1 Internal Symposium, Brussels, April 28-29 (1976). PA1 L. L. Richardson, "Use of Bleaching Clays in Processing Edible Oils", JAOCS, 55, 777 (1978). PA1 G. M. Clarke, "Special Clays", Ind. Minerals, Sept., 25 (1985). PA1 D. R. Taylor, D. B. Jenkins, "Acid-Activated Clays", Soc Mining Eng Of AIME, Transactions, 282 1901 (1988). PA1 R. L. Grim, "Applied Clay Mineralogy", pp 320-326, (1962). PA1 A. C. D. Newman, "Chemistry of Clays and Clay Minerals," pp 107-114, (1987). PA1 U.S. Pat. No. 1,397,113 (1921); Prutzman PA1 U.S. Pat. No. 1,579,326 (1924); Kauffman PA1 U.S. Pat. No. 1,642,871 (1927); Chappell et. al. PA1 U.S. Pat. No. 2,470,872 (1949); Secor PA1 U.S. Pat. No. 2,472,489 (1949); Pierce PA1 U.S. Pat. No. 2,484,828 (1949); Hickey PA1 U.S. Pat. No. 2,553,239 (1946); Christianson PA1 U.S. Pat. No. 2,563,977 (1949); Van Horn, Kahn PA1 U.S. Pat. No. 2,574,895 (1951); Stecker PA1 U.S. Pat. No. 2,671,058 (1952); Mickelson PA1 U.S. Pat. No. 2,872,419 (1959); Farnand PA1 U S. Pat. No. 2,892,800 (1959); Taipale PA1 U.S. Pat. No. 2,981,697 (1961); Mickelson, et. al. PA1 U.S. Pat. No. 3,617,215 (1971); Massaire, et. al. PA1 EPA No. 0,276,954 (1988); Alexander
The following patents relate to the production of acid-activated bleaching earths:
Generally, in the patents listed above, calcium bentonites are the source clays and high acid dosages, typically above 40-50 gms of 96% H.sub.2 SO.sub.4 /100 gms of dry clay, are required for maximum improvement in bleaching efficiency. The acid treated clay is invariably washed to remove soluble salts and entrained acid. See, for example, U.S. Pat. No. 1,397,113, U.S. Pat. No. 1,642,871 and the recently published EPA (0,276,954).
It is known to add citric acid to oils that are bleached with mixed attapulgite/calcium bentonite bleaching earths in order to enhance chlorophyll adsorption. Citric acid is not used to activate the clay.
U.S. Pat. No. 3,029,783 (Sawyer, et al), directed to an improved animal litter composition, describes an acid treatment, preferably using an attapulgite clay, which employs relatively low acid dosages without washing. The processing requires a calcination treatment at 700.degree.-1,000.degree. F. prior to the acid treatment and a second calcination at 750.degree.-1,100.degree. F. after acid treatment. The patent is not directed to the manufacture of a bleaching earth and we have found that the procedure does not lead to the preparation of a high efficiency bleaching earth.